The present invention relates to a motor drive control device that drives a three-phase DC motor. The present invention also relates to a method of operating the motor drive control device. More particularly, the present invention relates to a technology effective for properly starting up various types of motors under operating conditions where motor operations are performed in a wide range of temperature and power supply voltage.
A brushless three-phase DC motor is used as a spindle motor for rotating a magnetic disk of a hard disk drive (HDD). The spindle motor rotates the magnetic disk at a high speed, positions a read/write magnetic head close to the surface of the rotating magnetic disk, and moves the magnetic head in the radial direction of the magnetic disk to write information on the magnetic disk and read the information written on the magnetic disk.
When a brushless three-phase DC motor was subjected to drive control in the past, a Hall element was used to detect the positional relationship between a rotor (rotating part) and a stator (stationary part). Reverse motor rotation was prevented by determining an appropriate conduction start phase by selecting it from three coil phases in accordance with the detected positional relationship. However, when a rotor position detector based on the Hall element was incorporated in the motor, it was difficult to reduce the size of the motor. Hence, a sensorless DC motor was frequently used for the hard disk drive. If the positional relationship between the rotor and the stator is not properly determined at the beginning of rotation, such a sensorless DC motor may cause the rotor to rotate in a reverse direction.
A conduction start phase determination method described in Japanese Unexamined Patent Publication No. 2001-275387 sequentially supplies a short pulse current in a forward direction and in a reverse direction to the extent that a rotor does not respond to the field coil of each phase of a brushless motor, combines a voltage induced during a forward-direction supply with a voltage induced during a reverse-direction supply, which are both developed in a non-conducting phase, determines the polarity of the combined induced voltage, and determines a conduction start phase in accordance with the determined polarity. The induced voltage is a voltage that is induced on a field coil of a stator by the magnetic field lines of a magnet for the rotor in accordance with the positional relationship between the rotor's magnet and the stator's field coil. Currents flowing to the field coils of two out of three phases are sequentially alternated between forward direction and reverse direction to compare the resulting induced voltages. This makes it possible to determine which of the field coils of the two phases is closer to a pole of the rotor's magnet and determine whether the pole is an S-pole or an N-pole. Meanwhile, the reverse voltage (back electromotive voltage) of the motor is a voltage that is developed across a field coil of the stator in proportion to a revolving speed when the rotor's magnet rotates in a magnetic field of the stator's field coil. The reverse voltage of the motor is essentially different from the induced voltage of the motor.
FIG. 13 in Japanese Unexamined Patent Publication No. 2006-115599 and the description given with reference to this figure deal with motor startup control that includes a sequence of initial rotor position identification and conducting phase determination by a three-phase sense, motor drive by conduction, rotor movement verification by a three-phase sense, motor drive by conduction, rotor movement verification by a three-phase sense, and motor drive by conduction. The three-phase sense is accomplished by supplying a short pulse current in the forward direction and in the reverse direction to the extent that the rotor does not respond, subjecting an induced voltage developed in the remaining non-conducting phase to analog-to-digital conversion, and adding up the results of analog-to-digital conversion with an integrating register.
In the first three-phase sense for identifying an initial rotor position, the polarity of an induced voltage in each non-conducting phase of a total of three phases is determined to determine the conduction start phase in accordance with the determined polarities of the induced voltages in the three phases. When the motor is driven by initial conduction, the field coils of two phases that are determined as a conduction start phase by the first three-phase sense is energized for a relatively short predetermined period of time to subject the motor to a first initial acceleration. The second and third three-phase sense operations are performed to check for the reversal of polarity of an induced voltage concerning the next detection phase, which is determined from the conduction start phase.
FIG. 3 in Japanese Unexamined Patent Publication No. 2008-113506 and the description given with reference to this figure deal with a motor startup method that reduces the time required for motor startup by preventing the generation of noise during motor startup, which is determined by the sum of a three-phase sense detection period for initial acceleration described in Japanese Unexamined Patent Publication No. 2006-115599 and a conduction period for motor drive. In other words, the polarity of an induced voltage in each non-conducting phase of a total of three phases is determined by one three-phase sense operation, as is the case with the description given in Japanese Unexamined Patent Publication No. 2006-115599, and the conduction start phase is determined in accordance with the determined polarity of the induced voltage in each of the three phases. In subsequent PWM drive for conduction for motor drive, the induced voltage developed in a non-conducting phase is detected so that the conducting phase is changed in response to the detection of the peak of the induced voltage. Even after the conducting phase is changed, the peak of the induced voltage developed in a non-conducting phase during motor drive is continuously detected. The detected peak is then used to time a phase change for the purpose of accelerating the motor.